This invention relates to the flotation of coal having an oxidized surface to recover coal containing a lower percentage of impurities. In particular, this invention relates to the use of specific imidazoline compounds or imidazolinium salts to enhance the floatability of oxidized coal.
The natural process of "coalification" inherently deposits some non-combustible mineral matter in association with the combustible carbonaceous solids. Large fragments of coal together with non-combustible matter can be separated by screening or other conventional concentration techniques. Other methods more efficiently separate fine material intimately associated with the carbonaceous solids.
Froth flotation is sometimes employed to beneficiate finely-divided raw coal. Bituminous coals possess a natural hydrophobicity, which results in coals being readily floatable in the presence of a frother, such as methyl isobutyl carbinol, and a relatively mild collector, such as kerosene. However, coals of all ranks which have an oxidized or partially oxidized surface are much more difficult to float.
The loading of the oily collector is generally 0.05 to 1 kilogram per metric ton of coal feed for bituminous coals of intermediate or low rank, with the loading being relatively greater for the flotation of lignite and anthracite coals. However, good recovery of oxidized coals or lignite coals can only be effected at such high loadings of the collector that significant amounts of inert material are floated along with the combustible materials. Sun suggests in Trans. AIME, 199:396-401 (1954), that fatty amines can be utilized as co-collectors in the flotation of oxidized coals to effect enhanced recovery. However, even these amine collectors float substantial amounts of ash along with the coal and effect only partial recovery of combustible material.
Belov et al. report in an article in Abhandlungen der Akademie der Wissenchaften der DDR, Vol. 1, No. 4, pp. 225-33 (1976), that 2-aryl-2-imidazolines are useful as flotation agents for coal. This article is abstracted in Chem. Abstracts, 88:107, 116m (1978). However, the 2-aryl-2-imidazoline compounds have exhibited little activity in the flotation of oxidized coal in experiments detailed herein.